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BUREAU OF MINES 
INFORMATION CIRCULAR/1989 




Effects of Environmental Stressors 
on Vigilance Performance 

By J. C. Duchon and S. D. Hudock 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Mission: As the Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



Information Circular 9224 

Effects of Environmental Stressors 
on Vigilance Performance 

By J. C. Duchon and S. D. Hudock 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 



TNa.15 

.1(4 
.11 



Library of Congress Cataloging in Publication Data: 



Duchon, J. C. 

Effects of environmental stressors on vigilance performance. 

(Bureau of Mines information circular; 9224) 

Bibliography: p. 16. 

Supt. of Docs, no.: I 28.27:9224. 

1. Miners-Accidents. 2. Miners-Safety measures. 3. Miners-Job stress. 
I. Hudock, S. D. II. Title. III. Series: Information circular (United States. 
Bureau of Mines); 9224. 

-TN295rU4 [HD7269.M6] 622 s [363.1'19622] 88-600408 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Defining vigilance 3 

Theoretical models and specific effects of environmental stressors 4 

Theoretical models 4 

Specific effects 5 

Environmental stressors 6 

Vigilance performance under extreme heat conditions 6 

Vigilance performance under noise conditions 9 

Vigilance performance under vibration conditions 12 

Vigilance performance under adverse lighting conditions 13 

Conclusions 15 

References 16 

ILLUSTRATIONS 

1. Physiological arousal 4 

2. Three-factor taxonomic approach 11 

TABLES 

1. Maximum allowable noise exposure times 9 

2. Actual mining noise exposure 9 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


dB 


decibel 


in 


inch 


dBA 


decibel, A-scale 


kHz 


kilohertz 


op 


degree Fahrenheit 


km 


kilometer 


ft 


foot 


lx 


lux 


ft/min foot per minute 


min 


minute 


h 


hour 


pet 


percent 


h/d 


hour per day 


s 


second 


Hz 


hertz 


yr 


year 



EFFECTS OF ENVIRONMENTAL STRESSORS 
ON VIGILANCE PERFORMANCE 



By J. C. Duchon 1 and S. D. Hudock 2 



ABSTRACT 

The U.S. Bureau of Mines is conducting human factors research for the purpose of reducing accidents 
and improving the person-machine interface found in surface and underground mining operations. 
Miners are exposed to a variety of environmental stressors, e.g., extreme heat, noise, vibration, and 
adverse illumination, throughout the workday. Exposure to these environmental stressors has been 
noted in the literature to affect performance on vigilance tasks. Since impaired performance at vigilance 
tasks can lead to industrial accidents, further investigation of the effects of environmental stressors on 
human performance is warranted. A brief description of the environmental conditions present in the 
mining workplace is followed by a review of experiments dealing with the effects of environmental 
stressors on vigilance task performance. The applicability of past research to actual mining operations 
is considered. 

Engineering research psychologist. 

2 Safety engineer. 

Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN. 



INTRODUCTION 



The fit or misfit of inherent human ability and limita- 
tion within the work environment is the primary focus of 
human factors research. Specifically, this research is con- 
cerned with the design of tools, equipment, faculties, and 
working environments and their effects upon the individu- 
al's interactions with such systems and system components. 
There is concern when the conditions under which an indi- 
vidual must work cause a degradation in the health, safety, 
and performance of the worker. The primary goal of the 
U.S. Bureau of Mines human factors research program is 
to design working environments and machinery that opti- 
mize the health, safety, and performance of the mining 
work force. This report reviews research findings dealing 
with the effects that environmental stressors have on tasks 
that require sustained attention, which may directly or indi- 
rectly relate to accident causation. This review was under- 
taken to reveal possible sources of human error that have 
not been previously examined to provide a foundation for 
further research in the Bureau's human factors program. 

Safety experts agree that human error is involved in 
over 80 pet of industrial accidents. These accidents range 
in seriousness from minor slips and falls to nuclear power- 
plant disasters. Many of these human error accidents can 
be traced to impaired or lowered levels of alertness, i.e., 
not paying attention, missing warnings or cues, not watch- 
ing where one steps, not driving carefully, etc. It is com- 
monly observed that when individuals perform tasks that 
require sustained attention for extended periods of time, 
their performance in some situations will eventually begin 
to decline. This change in arousal or alertness has been 
referred to as the "vigilance decrement." Environmental 
stressors, such as extreme heat, noise, vibration, and ad- 
verse illumination, found in all mining sites, have been 
shown in previous research studies to alter the course of 
or interact with the vigilance decrement. 

This report focuses on the identification and char- 
acterization of those environmental stressors that have 
been shown to influence the vigilance decrement. This 
analysis will emphasize those conditions where sustained 
vigilance tasks appear to be the primary concomitant of 
alertness decrements that relate to decreased performance 
and safety. Environmental stressors are viewed as inter- 
acting factors that may effect the vigilance decrement. 
Lowered alertness or distracted attention can occur in 
other situations. For example, sleep deprivation or 
alcohol-drug usage can cause similar reactions. A job task 
that is too complex, too simple, or too long may, in and of 
itself, cause lowered alertness. Other factors that may 
influence alertness include circadian rhythms, work-rest 
cycles, motivation, mood, morale, individual differences, 
smoking, illness, amount of exercise or physical labor, 
incentives, and rewards (J). 3 



This report discusses the area of alertness and environ- 
mental stressors, but only in context of sustained opera- 
tions or vigilance, with special attention to mining occupa- 
tions, for several reasons. First, it is felt that the study 
of alertness is so encompassing that it would be more 
efficient to focus on a smaller problem area. The vigilance 
decrement is a clear, precise, and measurable variable that 
can be validly and reliably researched in a meaningful way. 
Second, most previous research on alertness has been done 
in the area of vigilance and sustained operations. It 
seemed an obvious step to relate this research to condi- 
tions in mining. Third, in a study performed by Hudock 
and Duchon (2), it was found that about one-third of the 
occupations in the surface mining industry were rated by 
two independent judges with mining engineering back- 
grounds to require high to extreme amounts of sustained 
attention-vigilance. The occupations were rated on the 
basis that (1) the tasks involved were prolonged and con- 
tinuous and lasted 30 min or more, (2) the tasks involved 
were boring and monotonous, and (3) there were few dis- 
ruptions within the job tasks. The surface mining occupa- 
tion ratings are shown as follows: 4 

High vigilance 

Auger Operator 

Barge Attendant 

Bobcat Operator 

Bulldozer Operator 

Cleaning Plant Operator 

Dispatcher 

Dragline Operator 

Drill Operator 

Dump Operator 

Fine Coal Plant Operator 

Forklift Operator 

Grader Operator 

Kiln Operator 

Power Shovel Operator 

Rotary BWE Operator 

Scalper Operator 

Scraper Operator 

Truck Operator 
Medium vigilance 

Belt Man 

Brakeman 

Cleanup Man 

Dimension Stone Cutter 

Drill Helper 

Laborer 

Oiler, Greaser 

Quarry Worker 

Silo Operator 



3 Italic numbers in parentheses refer to items in the list of references 
at the end of this report. 



■"Occupational titles are from U.S. Mine Safety and Health 
Administration, "Accident Data Analysis: A User's Guide," 1982. 



Stone Finishing 
Surface Miner 
Tipple Operator 
Washer Truck Operator 
Watchman 
Weighman 
Low vigilance 
Blaster 

Boom Operator 
Car Dropper 
Carpenter 
Coal Sampler 
Communication Repair 
Electrician 
Highlift Operator 
Machinist 
Mason 
Mechanic 
Supplyman 
Welder 
Yard Foreman 



Those surface mining jobs that were rated as highly vigi- 
lant had more than twice the accident severity rate 5 (96 
days) as those jobs that were rated as requiring low 
amounts of vigilance (45 days) for 1986. Two job tasks 
involving driving mining machinery are worthy of note. 
For scraper operators, the rate (152 days) was nearly three 
times the rate for the surface mining industry norm (54 
days) for the year 1986. Haulage truck operators had 
almost twice the rate (100 days) of the industry norm in 
the same year. In both situations, the operator was driv- 
ing an easy-to-operate vehicle at low speeds, in light traffic 
areas, in a high-noise environment, over sparse, uninterest- 
ing terrain, with few work breaks; the driver typically 
worked different shifts throughout the month. 



DEFINING VIGILANCE 



"Man is a poor monitor" (3). This statement was made 
in an opening of a book titled "Vigilance" to describe 
people's lack of ability in maintaining "watch" for pro- 
longed periods of time. There have been hundreds of 
articles and books that attempt to systematically study the 
conditions that relate to this performance degradation, 
often referred to as the "vigilance decrement." It is gener- 
ally accepted that in most instances performance tends to 
decay over time. Nevertheless, in spite of, and in some 
cases because of, technological advances, humans are 
required to perform vigilance tasks under conditions of 
sustained operations. Therefore, in industrial and military 
settings, it has been critical to understand the conditions 
that affect the speed and severity of the vigilance decre- 
ment. For instance, in the military, national security is 
dependent upon human accuracy in early radar detection 
and signal interpretation. Therefore, since World War II, 
the military has sponsored a great deal of research on the 
exploration of the conditions of optimal vigilance and 
sustained operations. 

In industrial settings, production, safety, and quality 
control are dependent upon tasks that involve sustained 
attention. In the mining industry in particular, missed cues 
or brief lapses of attention have been known to result in 
serious injury of the worker from roof falls, truck roll- 
overs, forward and backing-up vehicle collisions, etc. 
"Alertness" can be defined as "the ability to maintain 
optimal sensitivity to external stimuli" (4). By definition, 
the study of vigilance is concerned with the process of 
maintaining attention and alertness to stimuli over pro- 
longed periods of time. A vigilance task is "one that re- 
quires subjects to respond in some way to the occurrence 
of relatively infrequent and unpredictable (in time, space, 



or both) stimulus changes over relatively long periods of 
time" (5). These definitions should be considered only as 
general guidelines. It should be noted that quite different 
definitions exist (<5). For instance, McGrath (7) proposed 
that among other criteria, a vigilance task must contain 
these attributes: (1) The stimulus to be detected must be 
specified; (2) the ratio of nonsignificant to significant sig- 
nals should be high; (3) the signals should occur at random 
intervals; and (4) the response of the observer should have 
no effect on the probability of signal occurrence. These 
criteria limit tasks to simplified and controlled laboratory 
simulations, such as a visual signal detection. In contrast, 
other researchers have included a variety of tasks in their 
concept of vigilance, such as vehicle driving, radar opera- 
tion, air traffic control, industrial inspection, and the per- 
formance of anesthesiologists during surgery. A more 
comprehensive definition of vigilance situations was de- 
scribed by Warm (4): (1) The task is prolonged and con- 
tinuous, usually lasting longer than 30 min; (2) signals to 
be detected are clearly perceived by the observer when he 
or she is alerted to them, but may seem weak to most 
observers because they are not "compelling changes" in the 
observer's operating environment; (3) the signals to be 
detected occur infrequently and without forewarning; and 
(4) the observer's response typically has no effect upon the 
probability of occurrence of future signals. As the tasks in 
question become more like real occupational tasks, such as 
driving, definitions of vigilance and sustained operations 
must be more encompassing. 



'Accident severity is represented by the sum of the actual number 
of days lost plus the number of statutory days charged plus one-half the 
restricted workdays charged for each accident. 



However, there is a price to pay as definitions and tasks 
become more inclusive. It becomes more difficult to gen- 
eralize results from one situation to another. For instance, 
although it may be found that random bursts of noise may 
improve performance of a visual signal detection task in 
the laboratory, it may not be the case that a similar noise 
condition will aid a long-distance truck driver. There are, 
in fact, many seemingly inconsistent results in some of the 
literature (see the section "Vigilance Performance Under 
Noise Conditions"), which complicates drawing specific 
conclusions from past research. This problem is directly 
relevant to mining, since tasks of interest in the mining 
industry are more complex, such as haulage truck opera- 
tions, mechanical repairs, etc. 

Operational definitions of vigilance and corresponding 
vigilance decrement have been as varied as the research 
itself. Mackworth (8) has been credited with being the 
first to study sustained operation in a systematic way. 
During World War II, he was asked to study the problem 
of missed radar signals on antisubmarine patrol. Mack- 
worth devised a vigilance task called the clock test, which 
has been used in subsequent research. The clock was a 
simulated radar screen that showed a black pointer that 
moved around the otherwise blank clock, jumping 0.3 in 
each time. The critical signal was a double jump of the 
pointer, i.e., jumping 0.6 in. An observer would watch the 



clock for several hours at a time, indicating when the 
double jump was perceived. From these experiments, 
Mackworth concluded that accuracy of signal detection 
decreased most dramatically after about 30 min exposure 
and then gradually declined. Brief, planned interruptions 
would improve performance for approximately 30 min, but 
performance would then revert to previous low levels. The 
important point here is that the vigilance decrement (low- 
ered level of alertness across time) was operationally mea- 
sured and defined as a percentage of missed signals. 

At least three categories of methods of measurement 
have been used in vigilance research: (1) psychophysio- 
logical measures, i.e., electroencephalograph (EEG), elec- 
tromyograph, and galvanic skin response; (2) performance 
measures, such as signal detection (as in the clock test 
described above), errors of commission (false detections), 
changes in stimulus thresholds for positive detections, and 
response latency or response times; and (3) subjective 
measures of alertness and related mood states. Closer 
inspection of these measures shows that alertness is multi- 
domained. That is, alertness encompasses not only biolog- 
ical and chemical processes in the body but also corre- 
sponds to actual performance and to subjective mood 
states. Any complete conceptualization of vigilance and 
alertness, then, should take into account these aspects. 



THEORETICAL MODELS AND SPECIFIC EFFECTS OF ENVIRONMENTAL STRESSORS 



Several theoretical models have been put forth to 
account for the effects that environmental stressors have 
on performance. It should be noted that, while these 
theories do account for many of the findings, none are 
fully acceptable. It has been pointed out (9-11) that it 
would be unrealistic at this time to attempt to develop a 
comprehensive model that explains the seemingly incon- 
sistent findings of research. Briefly described below are 
the arousal model, attentional model, and control model, 
which attempt to explain the interaction between the 
vigilance decrement and environmental stressors. Also 
discussed are factors related to some of the stressors that 
help explain their specific effects on vigilance. 

THEORETICAL MODELS 

The most commonly cited model is the view that envi- 
ronmental stressors affect vigilance performance by activat- 
ing internal arousal mechanisms (12). According to the 
arousal or activation theory, performance rises with in- 
creases in physiological mechanisms within organisms, as 
measured by increased levels in heart rate, blood pressure, 
muscle potentials, skin conductance, and high-frequency, 
low-amplitude EEG activity (13). After a certain point of 
increasing activation, any further increase causes a drop in 



Optimal 



UJ 

o 
z 
< 

QC 
O 

U- 
QT 
UJ 
Q. 



Poor 




Low 



Moderate High 



AROUSAL 



Figure 1 .-Physiological arousal in inverted- U model. 



performance. One basic tenet is that there is some opti- 
mal level of activation where performance is at its best. 
Less or more arousal creates an understimulation or over- 
stimulation, which causes performance decrements. Figure 
1 shows this inverted-U relationship. The introduction of 
moderate levels of noise, for example, may improve perfor- 
mance by arousing the individual, but large amounts of 
noise for extended periods of time would hyperarouse or 
greatly fatigue a worker, causing decreases in perfor- 
mance. Poulton (12) discussed this model in great detail 
in relation to heat, noise, vibration, and isolation. The 
inverted-U model accounts for many vigilance research 
findings, but there is much that this model does not 
account for. 

Hancock and Pierce (14) posited an attentional homeo- 
static model to explain thermal effects on vigilance. They 
maintain that, as dynamic increases of body temperature 
occur, attentional resources are competed for, thereby 
draining attentional resources needed to maintain 
performance standards. This model better accounts for 
findings (as mentioned in the section "Vigilance 
Performance Under Extreme Heat Conditions") that 
indicate that automatic or highly skilled behaviors are less 
affected by stressors, since less attention is needed to 
perform the task. This theory would also apply to any 
other stressor, such as noise, that would tend to perturb 
attention. 

A less popular, but nonetheless reasonable, model to 
account for some of the effects of environmental stressors 
is the control or learned helplessness model. This model 
emphasizes the role of perception of personal control or 
lack of control and predictability over environmental 
events, such as noise or vibration. A perception of lack of 
control and of the unpredictability of noxious or unwanted 
stimuli will interfere with immediate and subsequent be- 
havior. A classic series of studies by Glass and Singer (15) 
demonstrated that the detrimental effects of intermittent 
noise peaking at 108 dB were attenuated when individuals 
were given the opportunity to switch off the noise, even 
though they did not choose to do so, or when the noise 
was more predictable. These studies show that the per- 
ception of the stressors is important and should be con- 
sidered when dealing with performance. Most studies that 
have demonstrated these effects have been in controlled 
laboratory settings. These results need to be verified in 
real-world settings. 

SPECIFIC EFFECTS 

Besides noise acting as an environmental stressor in a 
generic sense, as in the above models, noise can create 
performance decrements in subtle ways. Noise can mask 
auditory feedback that individuals use to signal certain 
responses. For instance, the click of typewriter keys, the 



sound of gears shifting, or the sound of high revolutions 
per minute in truck engines represent sensory feedback 
signals that workers often use for information about their 
performance. In intermittent and continuous noise, these 
indicators may not be heard. Also, acute partial hearing 
loss often occurs with exposure to noise, which would con- 
tribute to this problem. 

Related to the masking of auditory feedback is the 
masking of inner speech caused by external noise (16). An 
example of inner speech is the quiet rehearsal of a tele- 
phone number so as not to forget it. Tasks that involve 
short-term memory are particularly vulnerable to this. 
Several studies have shown that rehearsal of information 
is degraded because of noise (17). 

Finally, an often overlooked effect of noise is the an- 
noyance (16). Poulton (12) suggested that intermittent 
noise is more annoying than continuous noise and that 
unlocalized noise is more annoying than localized noise. 
Although difficult to measure and to objectively define, the 
detrimental effects of annoyance on performance, experi- 
enced over time, should not be underestimated. 

Vibration, especially at 5 and 7 Hz, has been shown to 
have activating effects on the central nervous system, sim- 
ilar to other stressors (18-19). Therefore, vibration could 
fall into the realm of activation theory for effects on per- 
formance. Vibration has, however, specific effects impor- 
tant in the consideration of performance at vigilance tasks. 
For instance, vibration has direct effects on visual percep- 
tion (18). Visual blurring has been documented in the 5- 
to 90-Hz range and seems to be related to specific fre- 
quencies (20). The loss of visual acuity correlates to the 
amplitude of vibration and peaks at 10 to 25 Hz (18). 

Whole-body vibration has been associated with a loss of 
a sense of limb position (21). In a manual-tracking exper- 
iment, a loss of isotonic control was demonstrated, owing 
to a decrease in the effectiveness of feedback information 
concerning limb motion and position. 

Finally, studies have shown that occupational vibration 
causes a diminution in grip force and fingertip sensation 
and tactile sensitivity (22). Each of the above effects of 
vibration could have important implications for perfor- 
mance of vigilance tasks. 

Many studies of the effects of extreme heat or cold on 
vigilance performance assume a cognitive deterioration at 
the vigilance task, as predicted by activation or attention- 
al resource theories. However, temperature-related de- 
creases in performance over time may have other causes. 
Cooling of the hand or fingers produces stiffness of the 
joints. A loss of dexterity occurs when the forearm is 
cooled, owing to an increase in muscle viscosity of the long 
flexors and extensors of the fingers (23). In general, stud- 
ies have shown that local cooling of the arm causes a sig- 
nificant decrement in manual dexterity, whether or not 
general body temperature is low. 



ENVIRONMENTAL STRESSORS 



The remainder of this report will discuss the ways in 
which four environmental stressors— extreme heat, noise, 
vibration, and adverse lighting-affect alertness in the con- 
text of sustained operations. How stressors affect alertness 
is of obvious significance in mining environments. Surface 
and underground miners are exposed to extreme environ- 
mental conditions such as dust, heat, cold, fumes and 
whole-body vibration. How and why these stressors affect 
vigilant performance is unclear. There are several theories 
that attempt to answer these questions, but as will be dem- 
onstrated in the following material, too many inconsis- 
tences still exist for a clear-cut unified theory. 

VIGILANCE PERFORMANCE UNDER 
EXTREME HEAT CONDITIONS 

Working in industrial settings where elevated tempera- 
tures are involved, such as close proximity to metallurgical 
furnaces, or exposure to extreme climatic temperatures, is 
the status quo in many job situations and is not easily 
altered for optimum human safety and performance. This 
section gives some background information on the thermal 
conditions present in the U.S. mining industry and then 
summarizes studies concerned with the effects on human 
sustained attention capabilities during exposure to elevated 
temperatures. 

Ambient temperature is a major environmental concern 
in many mining operations. Temperatures can range from 
115° F in the Southwest open pit copper mines to -45° F 
in the surface taconite mines of Minnesota. Mining oper- 
ations usually continue despite these climatic extremes. 
Underground operations do not have as wide a range of 
temperatures as surface operations. Underground ambient 
temperatures are usually in the range of 59° to 95° F, with 
relatively high humidity, depending on the depth of the 
mine and the type of deposit being worked. 

In addition to climatic conditions, certain geologic and 
operational variables contribute to the heat load in under- 
ground mines (24). The temperature of the wall rock may 
be elevated because of the radioactivity of the rock min- 
erals, proximity to igneous activity, oxidation of sulphide 
ore, or hot ground water. In addition to the native rock 
temperature, operational factors, such as blasting, oxida- 
tion of support timbers, heat produced by powered equip- 
ment, mine water and compressed air lines, and the adia- 
batic compression of ventilation air, contribute to the 
underground heat load. 

Job tasks also affect the thermal load of miners. Haul- 
age truck drivers and heavy equipment operators usually 
sit in enclosed cabs, which are often not temperature con- 
trolled. The tasks involved in mining, such as lifting, shov- 
eling, and pushing, are strenuous operations that inevitably 
raise body temperature, even when climatic conditions are 
normal. Metallurgical furnace attendants must work in 
close proximity to extremely high temperatures. Under- 
ground workers may work in ambient temperatures that 



are over 90° F with extremely high humidities. Other 
underground workers may work in conditions of relative- 
ly mild temperatures with strong airflow creating cool 
conditions. 

These are the basic thermal conditions to which work- 
ers in the U.S. mining industry are exposed. Conditions 
can change from day to day at surface operations because 
of climatic factors. Underground conditions are fairly con- 
stant from day to day but can change seasonally and within 
the different levels of a mine. How these thermal condi- 
tions affect the performance or safety of the miners on a 
daily basis is a matter of concern. A review follows on 
the effects of environmental temperature on vigilance 
performance. 

Mackworth (25) explored the effects of environmental 
temperature on the watchkeeping ability of subjects. The 
visual vigilance task used by Mackworth was mentioned in 
the "Defining Vigilance" section of this report. Subjects 
were given a single 2-h practice session with the clock test 
prior to performance at one of four elevated temperatures. 
In all conditions of the experiment, performance efficiency, 
as measured by missed signals or false alarms (reporting 
a signal when none occurrred), was reduced during the 
second hour of the task. Further, this vigilance decrement 
was magnified by the imposition of increased ambient heat. 
The study demonstrated the superiority of performance at 
79° F when compared with one lower and two higher effec- 
tive temperature (ET) 6 conditions, 70°, 88°, and 97° F. 
The study concluded that vigilance performance was better 
in warm environments, as compared with cool and hot 
environments. 

Bell, Provins, and Hiorns (27) investigated the effects of 
temperature on visual and auditory vigilance tasks during 
exposure to hot and humid conditions. In this study, the 
subjects were asked to monitor the movement of 20 sepa- 
rate dials in 5 different climatic conditions. Dry bulb (and 
wet bulb) temperature conditions, controlled in an envi- 
ronmental chamber, were 85° (76°), 109° (95°), 124° (99°), 
124° (109°), and 145° F (117° F), with an air velocity of 
50 ft/min. Each subject stayed within the environmental 
chamber as long as he or she was physically capable, for a 
maximum of 4 h. The mean exposure times for the tem- 
perature conditions stated above were 4 h, 187.5 min, 67.8 
min, 28.4 min, and 19.3 min, respectively. When the per- 
formance of the subjects was examined with respect to the 
proportion of signals missed to the signals given, no evi- 
dence was found of a change in vigilance under varying 
climatic conditions. However, it was found that body tem- 
perature was inversely related to performance, where the 
higher the body temperature, the worse the performance. 
That is, according to Bell, performance is a function of 
body temperature, not environmental temperature. A 



(r Thc ET scale is an index of perceived warmth considering mea- 
sures of dry- and wet-bulb temperatures and air velocity developed by 
Houghten and Yagloglou (26). 



major problem in interpreting these results is that the ex- 
posure time decreased as the environmental temperature 
increased, because the subjects left the chamber at the 
higher temperatures. 

Colquhoun (28) found a similar result in a study de- 
signed for analyzing the effects of differing frequencies of 
signal presentation and temperature on a vigilance task. 
Subjects responded to a light signal that was 30 pet 
brighter than the standard signal. Although there was a 
performance decrement across the length of the experi- 
ment, the three climate conditions, cool, warm, and hot, 
did not have a significant effect. This unexpected finding 
was conjectured to be due in part to the low frequency of 
signal presentation used. However, when subjects' oral 
temperature was analyzed, it was found that oral tempera- 
ture correlated with performance decrements (r = 0.71) 
for the cool environments but not for the warm and hot 
environments. 

Wilkinson, Fox, Goldsmith, Hampton, and Lewis (29) 
studied the direct effects of body temperature on vigilance 
tasks under different body temperatures while holding 
room temperature constant. The four body temperatures 
at which subjects were tested were 98.6° (normal body 
temperature), 99°, 100°, and 101° F. Body temperature 
was readily elevated by exposing the subjects to a hot, 
humid climate of 109.4° F and 100 pet relative humidity. 
Once the desired body temperature was reached, subjects 
were removed from the hot, humid climate and dressed in 
a vapor barrier suit. The performance tests were con- 
ducted in a room maintained at 98.6° F with a relative 
humidity of 20 pet. The subjects listened to a series of 
tones (1 kHz, 0.65 s) and signaled the occurrence of ran- 
domly placed longer tones (0.90 s). Performance at the 
vigilance task decreased as body temperature increased 
from normal body temperature to 99° F. An improvement 
in vigilance performance was manifested at 100° and 
101° F, the two higher body temperatures. This improve- 
ment was attributed to the arousing effects of heat. It was 
concluded from this study that performance on vigilance 
tasks is directly related to body temperature rather than 
environmental temperature. 

Benor and Shvartz (30) attempted to study the effects 
of cooling the surface-body temperature while core-body 
temperature remained constant. For this experiment, sub- 
jects were exposed to ambient temperatures that ranged 
from 86° to 122° F for periods up to 2 h. The subjects 
walked on a treadmill wearing impermeable suits. The 
impermeable suit did not allow the evaporation of perspi- 
ration or the dissipation of body heat, which resulted in a 
rapid elevation of core-body temperature. The test was 
replicated with the subjects wearing the impermeable suits 
that were equipped with built-in cooling systems, thereby 
keeping surface-body temperature, skin temperature, and 
heart rate constant. All of the experiments with the 
subjects wearing noncooling suits showed the same per- 
formance pattern in detecting an auditory signal: an ini- 
tial improvement for about 10 min, a leveling-off period, 
and subsequent deterioration. When the subjects wore 
the cooled impermeable suit, performance improved 



significantly; the detection rate leveled off after the initial 
adjustment period. The conclusions of this experiment 
were that detection rate is related to environmental tem- 
perature, not core-body temperature, and that detection 
rate is determined by heat stress as reflected by skin tem- 
perature and sweat rate, not heat strain as expressed by 
internal body temperature and heart rate. 

Loeb and Jeantheau (31) found that vigilance with 
respect to several different signal sources was unaffected 
by high environmental temperatures, except when high 
temperatures were combined with noise and vibration. 
However, the comparison of vigilance at the elevated tem- 
perature condition and the lower control temperature con- 
dition was confounded by possible differences in day versus 
night performance. Evidence suggests that signal reaction 
times are appreciably longer at night than during the day 
(32). Therefore, it was concluded that the effects of the 
elevated temperature condition may be masked by diurnal 
(daily) variations in the level of alertness. 

Bursill (33) investigated subjects' responses to light 
signals presented on the peripheral visual field while the 
subjects were also performing a centrally presented task 
under varying environmental temperatures. Performance 
on the two simultaneous tasks during 2 h exposure to tem- 
peratures of 65° and 95° F was compared. While attending 
to a psychomotor tracking task in the center of the visual 
field, each subject reacted to brief light signals occurring 
on the periphery of the visual field at irregular intervals. 
In the high-temperature condition, the peripheral signal 
was missed more often than in the low-temperature condi- 
tion. Signals on the lateral sides of the visual field were 
missed more often than those at the center of the field. 
These effects were found only when the centrally presented 
psychomotor tracking task placed heavy attentional de- 
mands on the subjects. The findings led Bursill to propose 
an explanation of heat effects on human performance in 
terms of "funneling of attention." Peripherally placed sig- 
nals may be missed at a higher rate when an individual is 
performing on a difficult central task compared with an 
easier central task. Bursill concluded that this condition of 
funneling of attention was intensified by exposure to heat. 

Another experiment was conducted by Colquhoun and 
Goldman (34) on the effects of elevated temperature on 
performance of a vigilance task. In this instance, body 
temperature was elevated by having the subjects walk on 
an inclined treadmill in hot and humid conditions (103° F 
dry bulb, 93° F wet bulb) for periods of 0, 10, 20, or 30 
min. The signal to be detected was a light signal that was 
approximately 30 pet brighter than the standard event sig- 
nal. Subjects responded to the occurrence of the signal 
at three levels of confidence-'maybe," "fairly sure," and 
"certain." The results showed that the detection rate was 
unaffected by changes in body temperature induced by 
various amounts of treadmill work immediately prior to 
the vigilance task. However, it was found that the percent- 
age of being certain of a signal detected was significantly 
greater following the 30-min treadmill task than following 
no treadmill work. The false report rate also increased 
with the lengthening of prior treadmill work. 



The combination of these two findings suggests that 
there was a decrease in the degree of caution exercised by 
the subjects in making a response. That is, the decision of 
the subjects to report a signal was presumably based on 
less adequate sensory evidence of signal occurrence as the 
subject's body temperature rose. It therefore appears that 
subjects exhibited strategy changes, as opposed to physical 
deficits, with regard to their performance. It was also 
found that for highly trained subjects, the rise in body 
temperature was considerable before any deterioration in 
performance occurred. This conclusion by Colquhoun and 
Goldman (34) is in line with that of Provins (55), who 
suggests that the effect of increased body temperature is 
an overall increase in the arousal of the subject. When the 
level of arousal exceeds a point that is optimal for perfor- 
mance of the task, then the increase in body temperature 
causes a deterioration in the performance of the task. 

Mackworth (25) found that the effect of increased tem- 
perature differed depending upon the prior experience the 
subject had with watchkeeping tasks. One group of sub- 
jects gained watchkeeping experience while performing 
naval lookout duty. This experience is analogous to the 
monitoring required in the experiment. The second group 
of subjects did not have previous watchkeeping experience. 
At temperatures of 70° and 79° F, there were no differ- 
ences between the experienced and inexperienced groups. 
However, at 88° F ET, those subjects with prior watch- 
keeping experience demonstrated superior performance 
over the inexperienced subjects' performance. It was con- 
cluded, therefore, that extreme temperatures have more 
effects when subjects are inexperienced. 

Hancock (36) reviewed the effect of skill on perfor- 
mance under environmental stress. He concluded that 
skillful performers or those performing less complex tasks 
are less vulnerable to adverse environmental conditions 
than are less skilled subjects, owing to the more automated 
nature of performance of the task by more skillful opera- 
tors. It was suggested that familiarity with the stressor 
may reduce the physiological impact on individual subjects 
while familiarity with the task reduces the behavioral 
impact of the stress. 

Hancock and Dirkin (37) investigated the effects of 
elevated head temperature upon performance of tasks, as 
opposed to raised whole-body temperature. In a study of 
central and peripheral visual choice reaction time under 
conditions of elevated head temperatures, slower but more 
accurate responses under the heat condition were found. 
In an experiment on the effects of elevated head tempera- 
ture upon the performance of simple mental addition (38), 
significantly more additions were accomplished under the 
heat condition. No significant effect on error rate was 
seen due to the heat condition. This study suggests that 
the processing rate in a behavioral task, such as mental 
addition, may be further abetted by a localized increase in 
head temperature. The differences between the results of 
the two studies may be due to the two types of tasks used, 
how the tasks were presented, and the difference in head 
temperature and temperature location. 



Some studies have attempted to study the effects of 
heat on vigilance performance in real-world situations. It 
should be cautioned that, while these studies may have 
more applicability to industrial settings, they lose a cer- 
tain amount of control over some potentially important 
variables. Romansky, Plummer, and Neumann (39) inves- 
tigated the effects of environmental stressors on perfor- 
mance of a simulated sustained driving task. The re- 
searchers evaluated the relative effects of a moderate, 
not extreme, level of heat and noise (90° F, 78 dBA) on 
human stress and fatigue using performance and physio- 
logical measures. The control condition was exposure to 
76° F and 55 dBA. Fatigue was defined as a group of 
phenomena associated with the impairment, or loss, of 
efficiency and skill, and the development of anxiety, frus- 
tration, or boredom. The higher environmental stress 
condition resulted in significant differences in heart rate, 
heart rate variability, and reaction time to a visual display. 
It was postulated that during sustained operation of driv- 
ing a vehicle on a roadway, driver performance can be 
negatively affected by moderate levels of environmental 
heat and noise stress. Elevated temperatures and noise 
levels create a stressful condition that consequently leads 
to subject fatigue and a deterioration in task performance. 

Mackie and O'Hanlon (40) investigated the combined 
effects of extended driving and heat stress upon arousal 
and driving performance. The subjects drove automobiles 
a total of 360 miles (580 km) over a 9-h period with a 
break of about 45 min at the halfway mark. Each of the 
subjects made the trip twice, once under self-selected 
"comfortable" conditions (approximately 67° F Wet-bulb 
globe temperature (WBGT)) and once at an elevated heat 
stress condition of 90° F WBGT. Physiological and perfor- 
mance measurements were obtained for each subject as 
were subjective ratings of alertness and fatigue. The per- 
formance measures included steering precision, vehicle 
speed control, driver errors, and performance on a second- 
ary vigilance task. One conclusion of the study was that 
exposure to the hot environment produced physiological 
signs of heat stress (such as increased sweat rate, increased 
oral temperature, overall elevation in systolic blood pres- 
sure, and greater heart rate variability) and signs of low- 
ered central nervous system arousal (as reflected by the 
lower average level of the EEG power ratio of alpha and 
beta waves) across time. After exposure to the heat for a 
period of about 5 h, the drivers reported more fatigue and 
decreased alertness than when in a comfortable environ- 
ment. During the heat condition, drivers had decreased 
steering control, increased lane shifting, and committed 
more technical driving errors than when driving at the 
comfortable condition. However, performance on the 
secondary vigilance task was inversely related to driving 
performance under hot conditions. This point raises a 
question regarding the applicability of secondary task per- 
formance measures in comparison with those for the pri- 
mary task. Also, using a secondary task may change the 
entire nature of the primary task. 



Ramsey, Burford, and Beshir (41) studied the effects of 
heat on safe work behavior. Unsafe behavior rate as mea- 
sured in this study is a function of the number of unsafe 
acts and the total number of observations made by impar- 
tial observers at two industrial plants. Among the con- 
clusions of this study was that ambient temperature had a 
statistically significant effect on unsafe work behavior rate. 
The relationship between unsafe work behavior rate and 
ambient temperature follows a U-shaped curve. The mini- 
mum unsafe behavior rate was shown to occur between 63° 
to 73° F WBGT. An increase in unsafe behavior rate 
occurs when the ambient temperature increases or de- 
creases outside of this range. 

Nearly all studies on the effects of heat on vigilance 
performance have found either no effect or a decrease in 
performance measures. Results that indicate an improve- 
ment in performance as a function of heat, suggesting an 
arousal mechanism, are rare. As will be shown in the next 
section, this is not necessarily the case for studies that have 
looked at noise as an environmental stressor. 

The question of whether it is environmental heat or 
inner-core temperature that relates to the performance 
decrement is still unanswered. However, the study by 
Benor and Shvartz (30), which found that by cooling the 
body's surface temperature the vigilance decrement was 
eliminated, should be further explored as an important 
research direction. 

Past research has also indicated that the task itself is an 
important determinant of the vigilance decrement under 
conditions of heat stress, e.g., event rates and task diffi- 
culty. Similarly, the skill level of the individual seems to 
have an effect. Finally, the idea that performance decre- 
ments are due to strategic changes of the individual has 
also been shown. 

VIGILANCE PERFORMANCE UNDER 
NOISE CONDITIONS 

In many work and nonwork environments, humans are 
exposed to sounds or noises, ranging from mild back- 
ground conversation to the 110-dB roar of a jet aircraft at 
1,000 ft. Noise pollution is prevalent in the mining indus- 
try where the use of common types of machinery guaran- 
tees that workers will be exposed to high levels of noise 
and vibration. The more severe of these conditions have 
been regulated against, allowing a maximum exposure time 
to different levels of noise. Table 1 presents maximum 
allowable exposure times for differing levels of noise in 
surface and underground mines, as regulated by the U.S. 
Mine Safety and Health Administration (MSHA) (and 
identical to U.S. Occupational Safety and Health Admin- 
istration standards) (42). The table also shows that the 
American Conference of Governmental Industrial Hygien- 
ists (ACGIH) suggests more stringent noise levels than 
MSHA levels. However, it is probable that miners are still 
being exposed to levels beyond these standards, as 
indicated in a 1981 MSHA study (43) (table 2). This 



section will suggest that noise, even at lower levels than 
what is considered physically harmful, may contribute to 
vigilance decrements. 

Table 1. -Maximum allowable noise exposure times 

Decibel MSHA ACGIH 

level, PDNE, TLV, Examples 

dBA h/d h/d 

65 Normal conversation. 

70 to 80 - - Roof signals. 

80 - 16 ft. 

85 - 8 ft. 

90 8 4 Trucks. 

92 6 - Rotary drill. 

95 4 2 Some mining 

machines. 

97 3 - ft. 

98 2 -3 - Shuttle car (load). 

100 2 1 ft. 

102 1.5 - Dozers. 

104 1 -1.5 - Wood planer. 

105 1 .5 ft. 

107 .75 - Continuous miner. 

108 .5 - .75 - Loading machine. 

110 5 .25 ft. 

115 <25 .125 ft. 

More than 115 NEP NEP ft. 

118 NEP NEP Drill, roof bolter. 

126 NEP NEP Large jet motor. 

130 NEP NEP Pneumatic hammer. 

140 ft NEP ft. 

NEP No exposure permissible. 

PDNE Permissible daily noise exposure. 

TLV Threshold limit value. 

x No example given. 

2 Maximum decibel, A-scale, that impact and impulsive noises 
reach (metal and nonmetal only). 

NOTE. -Dashes indicate no standard cited at this decibel level. 

Table 2.-Actual mining noise exposure (43) 



dBA 



h per 
8-h shift 



Shakeout 118 

Underground drill 116 

Crusher 107 

Mucker 107 

Surface drill 107 

Vibrating screen 103 

Bulldozer 102 

Front-end loader 101 

Load haul dump 101 

Chute 100 

Rod and ball mill 100 

Scraper and grader 100 

Continuous miner 100 

Longwall cutting machine 98 

Dragline and shovel 94 

Truck 93 

Shuttle car 93 

Kiln 90 



Among the most immediate reactions to loud noise are 
feelings of annoyance and discomfort (44). Studies have 
shown that increases in noise levels in industrial settings 
directly correlate with reports of annoyance and discom- 
fort (45). It has been found that workers who have been 



10 



chronically exposed to industrial noise complain of feel- 
ings of tiredness and fatigue and other disturbances, such 
as headaches, anxiety, disruption of sleep, irritability, and 
work and social conflicts (44). 

In a study of a film processing laboratory, noise was 
experimentally reduced from 99 to 89 dB in one room and 
was kept constant at 99 dB in another room. Comparing 
the results of this study, it was found film breakages were 
reduced in the quieter room (46). 

A few nonexperimental studies have clearly shown that, 
in some industrial settings, there is a direct correlation 
between noise levels and accidents. A study by Kerr (47), 
for example, investigated the accidents of over 12,000 
employees at 1 site. Of the 40 factors considered in the 
study, average noise level was one of the most highly 
associated factors. Noise level was positively correlated 
with accident frequency (r=0.42) but not with accident 
severity. Cohen (48) evaluated two plants for noise levels 
and accidents as well as other health-related variables. 
Five hundred workers situated in noisy work areas, defined 
as 95 dBA or higher, were compared with 500 workers in 
quieter areas, 80 dBA or less, over a 5-yr period. Cohen's 
data show that workers in noisy conditions had significantly 
more accidents than those in quieter conditions. 

Although these studies show that industrial settings may 
have noise conditions that could cause performance decre- 
ments, there are some studies that show performance 
improvements or no change in performance under noise 
conditions. It is, in fact, still uncertain as to what situa- 
tions produce positive or negative effects. It is also uncer- 
tain as to how the effects of noise interact with vigilance 
job tasks to produce these effects. 

Several reviews on the effects of noise on performance 
have been published in the past few years (11-12, 49-50). 
These reviews generally conclude that few generalizations 
can be made from previous studies because of inconsistent 
results. Consequently, it appears that it is impossible to 
predict the effects that particular types and levels of noise 
will have on specific tasks. However, several interesting 
models, such as the one presented in figure 2, have at- 
tempted to put some order into the seemingly conflicting 
results. This model suggests that, for example, when noise 
level is high and invariant (white noise) and processing 
demands are high, then performance will be degraded. 
However, it has been pointed out that this relationship is 
not so simple (50). Koelega and Brinkman (11) state that 
inconsistent results can be attributed to the complex inter- 
action of many factors. These factors include (1) the type 
of experimental design employed, (2) the intensity of noise 
used, (3) the comparison between different noise levels, 
(4) the frequency composition of the noise, (5) the type of 
noise (e.g., white noise, conversation, music, etc.), and 
(6) the scheduling of the noise stimuli (i.e., number of 
noise stimuli per time unit, time interval between noise 
stimuli and signal occurrence, and variability of interval 
between various noise stimuli). These factors coupled with 
the variety of tasks available (i.e., reaction time, cognition, 
discrimination, physical labor, driving, etc.), set up an 



almost infinite number of possible combinations. Gulian 
(51) states that, because of these factors, not only are re- 
sults totally unpredictable but also that one can only com- 
pare results from different studies in a general manner. 

In an effort to find generalizations regarding the effects 
of variable or intermittent noise on vigilance, Koelega and 
Brinkman (11) analyzed all studies from major journals 
since 1960 that had similar task characteristics-namely, 
simple sensory monitoring. Their hypothesis was that if 
task type is controlled for, consistent results may be found. 
Of the studies cited, 14 reported incremental and/or dec- 
rements effects of noise, while 15 of the studies showed 
no effects of noise. No pattern emerged from the anal- 
ysis. The authors concluded, therefore, that "we know 
nothing about variable or intermittent noise in vigilance 
experiments." 

In spite of all the confusion regarding the effects of 
noise on vigilant behavior, most authors would agree that 
vigilant situations are particularly sensitive to noise (52). 
Therefore, rather than make another attempt to draw gen- 
eralizations and conclusions from existing laboratory and 
field studies that would be applied to the mining environ- 
ment, the remainder of this section will discuss those vari- 
ables that have theoretical implications with predictive 
applications in these respects. The primary purpose is to 
delineate those factors that relate to and have potential 
effects on real working conditions. 

The largest body of vigilance research stems from an 
area referred to as the "Signal Detection Theory (SDT). 
While most of the work has been done in controlled labo- 
ratory situations, many of the principles gained from this 
research may be applied to real job tasks. In most vigi- 
lance tasks, subjects are asked to detect and then respond 
to auditory or visual signals for certain lengths of time. 
The clock test mentioned earlier is a popular example of 
a vigilance task. These laboratory tasks are designed to 
systematically measure people's ability to monitor or re- 
main vigilant at watchkeeping jobs. It could be argued 
that any physical task can be broken down into signal de- 
tection of some sort. Wiener (53), for instance, compares 
looking out of the windshield of a car with watching the 
gages of a control panel typical in vigilance tasks. 

According to SDT, there are two primary reasons why 
performance shows a decrement over time (54). If an 
individual experiences a decrement in his or her sensitivity 
to a stimuli, then the percentage of correct detections 
and/or reaction time will suffer. It has been found, how- 
ever, that the most consistent reason for a performance 
decrement is a change in an individual's criterion or strat- 
egy for responding (54-55). When individuals adopt a risky 
strategy, they will manifest a high percentage of false 
alarms (indicating a signal is present, when in fact there 
was none), which is usually accompanied by higher per- 
centages of correct "hits." When, on the other hand, sub- 
jects adopt a conservative approach, they will manifest a 
low rate of false alarms, which normally is accompanied by 
lower percentages of correct hits. 



11 



>- 



< 

o 

Ld 
GO 

O 



^0 



^ 








KEY 
■ Area of little empirical investigation 

: Equivocal results 

I Depressed performance efficiency 
T Improved performance efficiency 
— No change in performance efficiency 



Figure 2. Three-factor taxonomic approach to describe continuous noise effects on vigilance performance. 
(Adapted from Lysaght in reference 50.) 



12 



When a driver of a haulage truck detects a need to de- 
press the brake or turn the steering wheel, he or she has 
adopted a criterion for performing. According to the 
above definitions, therefore, a risky strategy would mean 
that a driver makes many adjustments in response to envi- 
ronmental stimuli, even though it may not be at all neces- 
sary (high false alarm rate). A conservative approach 
would mean that the driver will make fewer adjustments 
but also will not react to potentially important cues. This 
is what SDT would predict after sustained periods of driv- 
ing. The most consistent finding is that performance de- 
creases across a vigil owing to the adoption of a conserva- 
tive strategy for responding, as opposed to a decrease in 
sensitivity to stimuli. There are, however, some studies 
that have found performance decrements associated with 
a decline in perceptual sensitivity (56). Warm and Jerison 
(57) conclude that a sensitivity decrement appears to be 
related to tasks that involve a high rate of observing. 

A study by Hockey (58) shows the effects of vigilance 
and noise on a signal detection task and typifies much of 
the research in this area. Subjects were asked to watch the 
condition of three light sources by checking or sampling 
each light source one at a time by pressing one of a set of 
three corresponding buttons. The amplified valve noise 
was at 70 dBA in the quiet condition and 100 dBA in the 
noise condition. The task lasted approximately 30 min. A 
hit was defined as sampling a light source and immediately 
correcting the fault before taking another sample. A vigi- 
lance decrement was apparent in both the noise and quiet 
conditions, as evidenced by a drop in the percentage of hits 
in the second half of the vigil. The important point in the 
study was the assessment of the unsure hits defined as the 
correction of a fault in a light source after a second con- 
secutive sampling. Here, the subject "may be regarded as 
requiring further information before coming to a decision" 
(58). In the quiet condition, the number of unsure hits 
was three times that of the noise condition. There seemed 
to be a reduction of repeat responses in the noise condi- 
tion. It was concluded, therefore, that in the noise condi- 
tion, the tendency to make more definitive decisions (hits 
or misses) was increased. 

Similar results were obtained by Broadbent (52) and 
others. These studies indicate that performance decre- 
ments in quiet conditions have elements of uncertainty, in 
that performers tend to be unsure or uncertain of their 
mistakes, while in noise conditions, they may make as 
many mistakes but are more certain that they have not 
made a mistake. Noise tends to increase the confidence 
levels of their responses, in spite of their error rates (59- 
60). The important point is that, in quiet conditions, this 
level of uncertainty translates into behaviors that tend to 
offset a certain percentage of misses or mistakes. 

Another area that has been shown to affect vigilance 
performance in the presence of noise is priority of re- 
sponding. A 1954 vigilance study by Broadbent (52) 
showed that in a noise condition, subjects treated distinct 
parts of the experimental display differentially. Hockey 
(61) attempted to explore this phenomenon in more detail 
in a task designed to have features similar to driving a car. 



A tracking task was explained to subjects as having high 
priority in the overall task, as in steering an automobile. 
The tracking task was coupled with a task consisting of 
detecting lights arranged in a semicircle in front of, and 
equidistant from, the subject, as in detection of hazards or 
warning signals in a driving situation. Subjects were ex- 
posed to these tasks in either 70 or 100 dB white noise for 
40 min. In the loud noise condition, the detection of lights 
was poorer at the periphery and better at the center. Per- 
formance on the tracking task showed a vigilance decre- 
ment in the quiet condition but remained stable in the 
noise condition. It seems then that in the noise condition, 
subjects attended to high-priority or dominant aspects of 
their task (tracking task and central lights) at the expense 
of the other aspects. This effect has also been found in 
incidental learning tasks (62). It should be noted that in 
industrial accidents, many of the precipitating events are 
unexpected or unrelated to the dominant task at hand. 

A study by Sussman and Morris (63) offers further 
evidence of this effect. Subjects were exposed to a simu- 
lated driving task for 4 h under conditions of high and low 
task complexity and three levels of noise (based on noise 
amplitudes found in realistic driving situations). Vigilance 
decrements were found on various dependent measures 
over the 4-h vigil, including road position error rate, steer- 
ing wheel movements, and a simulated driving emergency. 
The interesting finding was that noise levels were not a 
significant variable for most of these measures. In fact, 
only the simulated driving emergency, which occurred 
randomly once per subject, was influenced negatively by 
high noise levels. Again, it appears that tasks that are 
not the dominant one, or are unexpected, are adversely 
affected by noise. 

The importance of using SDT as a basis for many of 
the vigilance studies has been to show that noise has its 
primary effects, not on the breaking down of sensory input 
directly, but in strategy changes that individuals make. 
The implications of these results are that accident causa- 
tion and risky behaviors may be increased under noisy 
conditions and that workers may change their strategy or 
criterion for safe behavior across a vigil. The confusing 
results found when attempting to look at specific task 
requirements should not mean that these variables should 
be ignored. On the contrary, they need to be carefully 
controlled so that their effects can be better understood. 
However, the promising results found in looking at strategy 
changes during vigilance and noise conditions are crucial 
in understanding behaviors that could lead to accidents. 

VIGILANCE PERFORMANCE UNDER 
VIBRATION CONDITIONS 

Exposure to vibration is a common condition in many 
industrial settings. The mineral industries are no excep- 
tion. In fact, there are many different forms of vibration 
that are prevalent in mining. Blasting of overburden and 
ore exposes surface miners to both air and ground vibra- 
tions of short duration. Plant workers are exposed to the 
constant vibration from crushers, vibrating screens, and 



13 



other pieces of plant equipment. Heavy-equipment opera- 
tors, such as haulage truck drivers and bulldozer operators 
at surface operations and continuous miner and shuttle car 
operators underground, are exposed to vibration for the 
majority of the workday. The surfaces over which the 
vehicles travel, as well as the vibrations emanating from 
the engines of the vehicles, contribute to the overall expo- 
sure to vibration. 

When vibration is transmitted to workers either through 
supporting structures, such as the plant floor or the vehi- 
cle's operator seat, it is referred to as "whole-body" vibra- 
tion. When vibration is through a handheld object, the 
vibration is called hand-arm vibration (64). Physical dis- 
orders of the spine, legs, arms, digestive system, and circu- 
latory system have been associated with exposure to whole- 
body vibration from operating vehicles. 

The International Organization for Standardization has 
developed a standard, titled "Guide for the Evaluation of 
Human Exposure to Whole-Body Vibration," (65), which 
considers the direction, frequency, intensity, and duration 
of vibration to which an individual is exposed. Vibration 
is then rated against three criteria. The first criterion is 
the reduced comfort boundary, which defines the vibration 
levels and durations that should not be exceeded if one 
wishes to preserve comfort. The second criterion is the 
fatigue-decreased proficiency (FDP) boundary, which en- 
sures preservation of working efficiency. The final criteri- 
on is the exposure limit (EL), which ensures the preserva- 
tion of health or safety. 

In a study of whole-body vibration exposures of mining 
machine operators (66), it was determined that about half 
of all surface mining machine operators were exposed to 
vibration in excess of the FDP level and about 15 pet were 
exposed to vibration in excess of the EL. The primary 
sources of vibration in surface mining were scrapers, bull- 
dozers, and loaders. For underground operations, about 
one-third of the machinery operators were exposed to 
vibrations in excess of the FDP level and about 12 pet in 
excess of the EL. The primary sources of vibration under- 
ground were shuttle cars and scoop trams. 

The effects of vibration on human health have been 
studied extensively in the aerospace and trucking indus- 
tries. These studies have focused primarily on the vigi- 
lance performance of commercial and military pilots and 
over-the-road truck drivers. This report deals primarily 
with the effects of vibration on the vigilance performance 
of individuals. 

It has been found that vibration is not always detrimen- 
tal to the individual. Several studies have found that expo- 
sure to particular levels of vibration may improve, or at 
least not hamper, an individual's performance on vigilance 
tasks (67). Holland (68), for instance, assessed perfor- 
mance at a compensatory tracking task during 6 h of con- 
tinuous exposure to random vertical vibration. Random 
aperiodic vibration was used since it more closely approxi- 
mates the vibrations reported in operating motor vehicles. 
Each of the four vibration conditions tested led to a de- 
cline in tracking performance compared with the control 
conditions. A secondary vigilance task of responding to 



warning light signals was also evaluated. However, the 
warning light task was not significantly affected by the four 
vibration levels tested. The average time to respond to 
warning signals increased over the 6-h testing period. In 
addition, it was observed that the vibration in some in- 
stances served to keep the subjects alert. This finding was 
supported by the comments of the subjects. 

Gray, Wilkinson, Maslen, and Rowlands (69) studied 
the effects of a 3-h exposure to vertical vibration at 5 Hz 
on the performance of four separate tasks: audio vigi- 
lance, visual search, compensatory tracking by hand, and 
handwriting. During the audio vigilance task, subjects 
listened to short tones (0.5 s at 600 Hz) given at 2-s inter- 
vals. The signal to be detected was a shorter tone (0.4 s 
at 600 Hz) presented on the average once every 1.5 min. 
The subjects responded by pressing one button signifying 
the detection of the signal and by pressing another button 
signifying their level of confidence that the signal was 
detected correctly. The number of signals detected, the 
number of false detections, and the levels of confidence 
were recorded for each of eight subjects. The overall 
difference in performance of the vigilance task was moder- 
ate but was not statistically significant. It was found that 
vibration did impair performance over time only when the 
subjects had no knowledge of test results. The perfor- 
mance at the other three tasks— visual search, tracking, 
and handwriting— was impaired by vibration but again the 
impairment was not statistically significant. 

Poulton (70) suggested that the positive effects of vibra- 
tion depend not only upon the quality of vibration but also 
on the nature of the task itself. Individuals experiencing 
whole-body vibration at frequencies between 3.5 and 6 Hz 
can decrease amplitude by tensing their trunk muscles. 
Poulton suggests, therefore, that individuals naturally tense 
their muscles to lessen the effects of vibration. During 
boring and monotonous tasks, the tensing of muscles can 
serve as an alerting mechanism. During an interesting, 
short, and challenging task, this would interfere with per- 
formance. Further, as an individual's alertness falls and 
his or her trunk muscles relax, the shoulders vibrate again, 
thereby alerting the individual. Such a reaction does not 
occur at frequencies above or below this range. These 
results have only been found in vigilance tasks of duration 
of up to 3 h. Whether these benefits occur throughout a 
full workday remains to be seen. 

VIGILANCE PERFORMANCE UNDER 
ADVERSE LIGHTING CONDITIONS 

The lighting conditions in underground mines are often 
considered to be of the poorest quality of any industrial 
setting. The illumination is completely from artificial 
sources, which creates a problem with the quality or quan- 
tity of light available. The main source of light under- 
ground is the battery-powered cap lamp. However, the 
cap lamp creates problems with glare, loss of peripheral 
vision, and lighting of the immediate task only. A series of 
general recommendations about improving mine lighting 
underground is presented by Trotter (71). Surface mines 



14 



often operate two or three shifts that require people to 
work at night. Again, similar problems arise because of 
artificial light sources. Haulage roads are usually only 
illuminated by vehicle headlights. The difference in light- 
ing levels from one section of the mine to another, both on 
the surface and underground, may itself cause vision- 
related problems among the mine workers. 

There is very little literature on how differing amounts 
and types of illumination affect an individual's vigilance 
performance. However, there have been numerous reports 
on how certain levels of illumination can cause eye strain 
and eye fatigue and can affect a person's visual acuity, 
especially with respect to video display terminal (VDT) 
operations. An excellent review of occupational stress in 
VDT operations was published by Dainoff (72). The 
studies reviewed included an assessment of visual fatigue 
and/or performance, musculoskeletal symptoms, and oper- 
ator attitudes toward job demands and quality of working 
life. The studies showed that increased VDT work time 
related to higher levels of boredom, fatigue, monotony, 
and physical stresses, which can result in lowered job 
performance. Dainoff concludes that work at VDT's raises 
the incidence of physical disorders and lowers job satisfac- 
tion in many situations and may be attributed to a number 
of factors: visual, postural, environmental, and task organ- 
ization. The visual factors include the display attributes of 
the video display screen as well as the contrast, glare, and 
illumination levels of the environment. Postural factors 
are dependent upon type of furniture used and the VDT 
location. Environmental factors of temperature, humidity, 
and presence of air conditioning may affect individual 
performance. Task organization factors such as level of 
cognitive complexity, perceived control over daily activities, 
and the length of time operating a VDT, are relevant 
characteristics to visual performance at VDT tasks. 

There have been studies that have looked at the effects 
of performing a central task on the amount of illumina- 
tion necessary to see peripheral stimuli. Although these 
studies are not directly related to vigilance performance, 
they may have practical significance to vigilance tasks 
where the performance on peripheral tasks has been 
shown to deteriorate. Leibowitz and Appelle (73) investi- 
gated the change in luminous energy required for detection 
of peripheral stimuli when a centrally presented task was 



more demanding. It was found that higher levels of lumi- 
nance were needed as the peripheral stimuli were located 
farther from the central task. This finding is important to 
the mining industry since lighting conditions are generally 
poor underground and in surface mines during the even- 
ing and night shifts, when the majority of light is supplied 
by point sources either mounted on mining vehicles or on 
the miners' hard hats. Any hazard that occurs on the 
periphery of an individual's vision may not be noticed 
before an accident or injury occurs. 

Several studies have been conducted that consider the 
relationship between illumination and mining accidents at 
underground operations. Van Graan, Greyson, Viljoen, 
and Strydom (74) conducted illumination surveys of 19 
underground gold mines. It was stated that when a task is 
visually exacting and vigilance is necessary, illumination 
must be good if the work is to be done in an efficient 
manner and without strain. If the task is visually simple, 
then comparable efficiency can be achieved at much lower 
levels of illumination. The mean light intensities found 
ranged from 8 be in haulageways to 82 lx at electrical sub- 
stations. This wide range in illuminance may result in an 
impairment in the ability of a miner to adapt to conditions 
rapidly and maintain some degree of visual acuity in 
underground operations. The resulting poor vision may be 
a safety hazard to the individual miner as well as to any 
nearby coworkers. 

Martin and Graveling (75) considered the loss of 
peripheral visual awareness when an area is illuminated 
solely by a miner's cap lamp. Since the cap lamp is a 
point light source and is highly directional, being mounted 
on the front of a miner's hard hat, the peripheral sur- 
roundings around a miner are not illuminated by the cap 
lamp. This in turn may lead a miner to fail to recognize 
nascent hazards in the vicinity. The experimenters mea- 
sured peripheral vision in four lighting conditions: cap 
lamp only, cap lamp with 5-lx background illuminance, 5- 
lx ambient illuminance only, and a control condition of 
450 lx. For all subjects, the area of peripheral vision was 
greater with the combination of cap lamp and background 
illumination than with just the cap lamp. This improve- 
ment in peripheral field of view averaged 9° greater on the 
temporal side in the horizontal plane. 



15 



CONCLUSIONS 



Four environmental stressors-extreme heat, noise, ad- 
verse illumination, and vibration-have been evaluated in 
relation to their effects on performance on vigilance tasks, 
especially in industrial settings. Relatively few studies have 
been done on the effects of illumination and vibration on 
vigilance tasks. The studies that have been conducted on 
heat and noise have often been contradictory or limited in 
generalizability by the use of laboratory settings and vari- 
ance in methodologies. It may be concluded that, in many 
instances, heat and noise do cause a worsening of the 
vigilance decrement. It should be noted, though, that 
some studies have found no effects of heat and noise on 
performance, while a few studies even found that heat and 
noise improved performance. These inconsistencies lead 
to the conclusion that any particular situation involving an 
interaction between an environmental stressor and a vigi- 
lance task should be looked at as a unique situation with 
unpredictable results. 

It should be a major concern that illumination and 
vibration have not been thoroughly examined. The consid- 
eration of noise without vibration may be misleading since 
noise does not usually occur separate from vibration in 
most industrial settings. While no one would dispute the 
importance of proper illumination for ideal performance, 
the effects of lesser amounts of illumination on vigilance 
have not been thoroughly explored. This lack of informa- 
tion is especially important in the mining industry, where 
work is often done outside, during evening and night shifts, 
or underground, where the primary light source is a nar- 
row band of light. 

Several basic points derived from the literature are 
outlined below and should be considered for future work 
in this area: 

1. Most studies confirm the notion that extreme heat 
and noise have a negative effect on vigilance tasks. 



However, just how much of the stressor is needed before 
an effect is manifested is unknown but may depend upon 
the task conditions and ability levels of the individuals. 

2. The effect may not be manifested in the primary 
task itself. The decrement in performance may be mea- 
sured on peripheral or secondary tasks or in reaction to 
unexpected events that are unrelated to the primary task. 
The primary task may even show an improvement. 

3. In order to study particular occupations for vigilance 
decrements, experiments need to be as realistic to the task 
as possible. Many of the differences in results have, at 
least in part, been due to slight differences in 
methodology. 

4. Baseline data are needed for the effects of these 
environmental stressors, especially for illumination and 
vibration. 

5. Future research should be concerned with the inter- 
action of these stressors among themselves. In most in- 
dustrial settings, noise and vibration are associated; heat 
is often accompanied by noisy conditions and poor 
illumination. 

As mentioned in the "Introduction" section, it was found 
that one-third of the occupations in mining were rated as 
having the major components of vigilance- type tasks. One 
occupation in particular, driving, has been explored in 
previous studies and has shown interactions with heat and 
noise on measures of performance. It is likely, therefore, 
that studying similar mining tasks may prove worthwhile in 
finding and eventually controlling stressors that affect the 
health and safety of miners. 



16 



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